Metal-bonded cubic boron nitride crystal body

ABSTRACT

Cubic boron nitride crystals have been bonded together into an abrasive body with an alloy bonding medium. The preparation of this abrasive body includes selection of an alloy (using separate metals to be alloyed in situ or pre-formed alloy) to be used comprising a first metal selected from the group consisting of aluminum, silicon, cobalt, nickel, chromium, molybdenum, tungsten, vanadium, niobium, tantalum, titanium, zirconium, lanthanum and other rare earth metals with any metal that, when molten, will wet said first metal and form an alloy therewith that is homogeneous on solidification, said alloy having a melting point below about 1500*C.

United States Patent Fontanella [111 3,868,234 1 Feb.- 25, 1975 METAL-BONDED CUBIC BORON NITRIDE CRYSTAL BODY [75] inventor: Ethel L. Fontanella, Burnt Hills,

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Nov. 1, 1972 [21] Appl. No.: 302,821

Related U.S. Application Data [63] Continuation-impart of Ser. No. 158,991, July 1,

1971, abandoned.

[56] References Cited UNITED STATES PATENTS 8/1960 Wentorf 51/307 6/1964 Bovenkerk et a1 51/307 3,212,852 10/1965 Bundy ..51/307 3,233,988 2/1966 Wentorfet a1 ..51/309 Primary Examiner-Donald J. Arnold Attorney, Agent, or FirmLeo l. MaLossi; Joseph T. Cohen; Jerome C. Squillaro [57] ABSTRACT Cubic boron nitride crystals have been bonded together into an abrasive body with an alloy bonding medium. The preparation of this abrasive body includes selection of an alloy (using separate metals to be alloyed in situ or pre-formed alloy) to be used comprising a first metal selected from the group consisting of aluminum, silicon, cobalt, nickel, chromium, molybdenum, tungsten, vanadium, niobium, tantalum, titanium, zirconium, lanthanum and other rare earth metals with any metal that, when molten, will wet said first metal and form an alloy therewith that is homogeneous on solidification, said alloy having a melting point below about 1500C.

6 Claims, No Drawings METAL-BONDED CUBIC BORON NITRIDE CRYSTAL BODY BACKGROUND OF THE INVENTION This is a continuation-in-part of U.S. Pat. application Ser. No. 158,991 Fontanella, filed July 1, 1971, now abandoned and assigned to the assignee of the instant application.

This invention relates to improvement in the unification of individual crystals of the cubic form of boron nitride (CBN) into a coherent conglomerate mass, e.g. an abrasive body or compact. Also, the provision of individual CBN crystals covered with a well-bonded alloy layer for inclusion in various abrading systems is an important aspect of this invention.

The preparation of CBN is described in U.S. Pat. No. 2,947,617 Wentorf, Jr. and prior art methods of preparing CBN compacts have been described in U.S. Pat. No. 3,136,615 Bovenkerk et a1. and U.S. Pat. No. 3,233,988 Wentorf et al. The Wentorf 167 patent is incorporated by reference.

U.S. Pat. No. 3,553,905 Lemelson discloses composite cutting and grinding tool structures having a metal base molded or cast to shape with abrasive bits embedded in the surface thereof. In the case of boron nitride (undoubtedly the cubic form in view of reliance on the abrasive property thereof) a layer of CBN bits is disposed against the surface of a casting or injection mold after which metal, such as steel is cast in situ against the layer of bits so that the bits become locked within the surface stratum f the tool. No mention is made of any specific steel. The term steel," of course, without further descriptive language indicates that this ferrous alloy contains iron plus carbon (less than 1.1% by weight) plus minute amounts (less than 1% by weight in the aggregate) of silicon, phosphorous, sulfur, manganese and/or aluminum.

SUMMARY OF THE INVENTION It has been found that CBN crystals can be bonded together using an alloy as the metallic bonding medium therefor. In the preparation the selected alloy may be generated in situ or pre-formed alloys may be used. The alloy may be a two, three, four (or greater number) component system. Tool inserts may be formed in preselected configurations with facilities provided for clamping or otherwise fastening the tool insert on a lathe tool, which in turn is held in a suitable holder.

Pressures during preparation may range from subatmospheric to pressures greater than atmospheric but less than about 1 kilobar' (kb). The cubic boron nitride content may extend to less than 70% by volume of the total amount of metal and CBN crystals.

DESCRIPTION OF THE PREFERRED EMBODIMENT It has been found that by employing slow heating and cooling at low pressures, tenacious alloy coatings may be applied to CBN crystals serving as a matrix to hold these crystals together as an abrasive body.

Nickel aluminum alloys and cobalt aluminum alloys have been found to be particularly effective.

Examples are set forth below of specific experiments conducted using nickel-and cobalt alloys prepared in situ and using a molybdenum pre-formed alloy. In addition to alloys of these metals, alloys may be used wherein a first metal is selected from the group consisting of aluminum, silicon, chromium, tungsten, vanadium, niobium, tantalum, titanium, zirconium, lanthanum and other rare earth metals and together therewith any metal is used which, when molten, will wet said first metal and form a two, three, four (or greater) component alloy system therewith that is homogeneous on solidification, said alloy having a melting point below about 1500C. Functionally, the alloy system used as the bonding medium should be capable of reducing any thin B 0 glass film that might be coating the CBN. Utilization of at least one of the aforementioned specifically mentioned metals as a component of the alloy system assures the presence of an element that forms a horide or nitride but the ideal alloy system is one that exhibits a finite but limited reactivity with CBN. Alloy systems in which too much of an active metal (one that forms a highly stable nitride and/or boride) is present may convert too much of the CBN to the active metal boride or nitride.

Thus, when very small (e.g. 1-5 micrometer) crystals of CBN are to be bonded together it is important either to select an alloy system that does not react to too great an extent with CBN (or else many crystals may be destroyed) or to select an alloy system that forms a diffusion zone with the CBN of material that introduces highly desirable properties to the system. Also, this invention enables the covering of individual CBN crystals with metal that is bonded to the CBN crystal through a diffusion zone. Such alloy-covered CBN crystals may be placed in metal-bonded, resin-bonded or ceramicbonded abrasive systems for the preparation of various cutting and polishing tools (e.g. flexible belts).

Except, perhaps, in the case of tool steels, the component of lesser (or least) concentration should be present at least in a quantity ranging from 1 to 5% by weight of the alloy system although larger quantities may be used.

The ability to utilize various tool steels as a matrix for CBN compacts is a particularly important aspect of this invention. Such CBN compacts may be age hardened for maximum performance in machining, drilling, grinding etc. Thus, tool steels may readily be selected that have various useful specific properties to be made available in the finished article, e.g. a CBN/tool steel tool insert. Tool steels may be selected such that the tool steel in a CBN/tool steel composite may be oilhardened or air-hardened and may be made shock resisting, highly wearresistant, highly resistant to heat softening, etc. by treatments at temperatures significantly below 1500C. There are chromium, tungsten and molybdenum tool steels available for various combinations of properties [Metals Handbook, 8th Edition 1961, Vol. 1, Tool Steels pp 637 et seq.].

In contrast to prior art high pressure, high temperature processes for preparing CBN abrasive bodies, the process of the instant invention may be carried out under vacuum, at atmospheric pressure or under greater, but relatively low, pressures (less than 1 kb). Since the CBN abrasive body, when used for metal cutting, will be used at temperatures of the order of 800l0O0C, using an alloy system having a high melting point is definitely advantageous in this respect. However, the process is essentially one of liquid phase sintering and since CBN reverts to hexagonal boron nitride at about 1650C, alloys having melting points in excess of about 1500C should not be used.

Both heating and cooling should, preferably be gradual with the cooling period being roughly one-half as long as the heating period.

EXAMPLE I A small number of CBN crystals together with nickel powder were loaded into a Ta boat. The assembly was heated up slowly while maintained in a vacuum of about 4 X 10 torr (1 torr 1 mm of mercury). When the Ni-Ta eutectic was reached, the temperature was held for about 5 minutes and then the system was cooled slowly by reducing the temperature in increments of about 50C every 3-4 minutes. The vacuum was not broken until the boat had cooled. Total time for heating and cooling is about 45 minutes. The system was allowed to cool at atmospheric pressure. The Ni-Ta alloy that had been generated had wetted and adhered to the CBN crystals. The coated CBN crystals were bonded together in the alloy matrix.

EXAMPLE II A powder mixture of Co (with 0.04 wt. Ti) and CBN crystals (100/120 U.S. Sieve size) were placed in a tantalum boat. The system was heated slowly in a vacuum (4 X torr) to the eutectic point of the Co-Ti alloy. The temperature was slowly reduced to room temperature. Some of the powdered mixture was unable to melt and the crystals adjacent thereto were not coated. However, on one side of the Ta boat many CBN crystals were exposed to the molten alloy and were covered with the Co-Ti alloy. These CBN crystals were firmly bonded together.

EXAMPLE llI Pure nickel wire (1.1 wt. of total metal) plus a 1 inch length of 0.005 inch dia. Cu (with 0.56 wt. Fe) wire together with CBN crystals were placed in a Ta boat. The CuFe wire was introduced to lower the melting point of the nickel. The system was gradually heated in a vacuum (4 X 10' torr) over a period of about minutes to the eutectic of the Ni-Cu-Fe-Ta alloy. When the Ta boat began to react, heating was stopped and the system was permitted to cool. Microscopic examination established that the CBN crystals were well wetted and had been bonded together.

EXAMPLE lV Example 1 was repeated using cobalt powder, CBN crystals and 21 Ta boat. About 1-5 wt. of the Ta reacted with the Co and the Co-Ta alloy bonded well to the crystals forming a CBN alloy-bonded compact.

EXAMPLE V A powder mixture of 4 wt. Co and a mixture of 99 wt. Ti 1 wt. Ta was placed in an inert boat with 100/120 U.S. Sieve size CBN crystals. The mass of CBN crystals were sintered sufficiently to show that the alloy formation had reacted with the surfaces of the CBN crystals and that with higher temperature (enough to reach the eutectic) the desired liquid phase sintering would have been achieved.

EXAMPLE VI A quantity of 2 Co 98 Ta (by wt.) powder together with a few CBN crystals was enclosed in a highly refined aluminum oxide container. The enclosure and the mixed contents were heated in air with a hydrogen flame. The metal powder alloyed and bonded the crystals together.

EXAMPLE Vll EXAMPLE Vlll A 0.005 inch thick tungsten boat was loaded with two strips of Co, several CBN crystals and a powdered mixture of WC and 13 wt. Co. The system was heated over a period of 35 minutes until melting occurred. The system was permitted to cool over a period of about 15 minutes. Many CBN crystals were covered with a wellbonded alloy coat.

EXAMPLE IX Several CBN crystals and some chips of M-2 tool steel were loaded into an alumina crucible. The crucible and contents were heated over a period of minutes to the melting point of the tool steel. The crucible was cooled over a period of 30 minutes. The CBN crystals were bonded together; the abrasive body having been used to cut a glass slide without any tearing out of CBN crystals therefrom.

An important benefit of this low pressure, liquid phase sintering process is the capability afforded for casting tool inserts. It is merely necessary to construct a refractory crucible (e.g. high purity alumina) having the bottom interior thereof in the shape of the desired tool insert, to provide biasing means, e.g. a piston to urge the mass into the crucible mold and to heat the system (e.g. by resonant frequency [RF] heating) to the eutectic. Preferably, the heating should be accomplished in an inert atmosphere to minimize oxide formation.

EXAMPLE X An alumina crucible was loaded with a layer of CBN crystals and a mixture of Ni (3 parts) and Ta (1.75 parts) powders. Additional layers of Ni/Ta powder and CBN crystals were added to fill the crucible. A top layer of CBN crystals was added and packed down. A light weight piston was used to force the powder down during heating. The system was heated to 1400C with RF heating in an argon atmosphere. The amount of charge was insufficient to make a 4-cornered insert as planned, but enough material was present to form one side and two corners of the insert out of alloy-bonded CBN crystals.

EXAMPLE X] A mixture of 39 wt. Mn, and 61 wt. Ti was placed in an alumina crucible with several CBN crystals as in Example X. The system was heated to 1450C. The heater burned out before the entire mass of metal could melt. CBN crystals in contact with metal where melting had occurred were coated.

EXAMPLE xu A mixture of 2 wt. Co and 98 wt. Cu together with several CBN crystals were placed in an alumina crucible as in Example X. RF heating to about 1350C resulted in melting. Upon cooling the CBN crystals were bonded together by the alloy.

EXAMPLE XIII A mixture of 50 wt. Ti powder and 50 wt. Co powder together with several CBN crystals were placed in a tantalum boat. The system was heated until melting occurred. The molten alloy wetted the CBN crystals and upon cooling the coated metal crystals were bonded together.

EXAMPLE XIV A powder mixture of 50 wt. Ni and 50 wt. Ta together with a few CBN crystals were loaded into a tantalum boat as in Example I. The system was heated until melting occurred and the alloy wetted the CBN crystals. Upon cooling the CBN crystals were bonded together.

The following examples were performed using a series of prepared binary nickel alloys to bond together a plurality of CBN crystals. Each of the metals added to the nickel was present in the amount of atomic percent. The vacuum employed in Example 1 was 10 torr. In all other examples a hydrogen environment was maintained during the reaction.

Compacts prepared according to this invention were cut and polished. Microscopic observation established that diffusion zones (indicating the formation of intermetallics or solid solutions) had been produced over the surfaces and in fine crevices of the CBN crystals between the crystals and the alloy or tool steel mass. The application of force to a CBN crystal in the bonded system caused fracturing of the crystal, but the bonded periphery thereof remained intact and firmly affixed to the alloy matrix.

What I claim as new and desire to secure by Letters P t t eit Qni tates is:

1. An abrasive body consisting essentially of cubic boron nitride crystals and an alloy matrix, the alloy matrix comprising a first metal selected from the group consisting of aluminum, silicon, vanadium, niobium, lanthanum and other rare earth metals and together therewith any metal that, when molten, will wet said first metal and form an alloy therewith that exhibits finite, limited reactivity with cubic boron nitride and is homogeneous on solidification, said alloy having a melting point below about I500C and each component of said alloy consisting of at least 1 percent by weight of said alloy and said alloy matrix constituting at least 30% of volume of said abrasive body.

2. The abrasive body of claim 1 wherein the body is formed as a tool insert.

3. The abrasive body of claim 1 wherein the alloy matrix is a nickel aluminum alloy.

4. The abrasive body of claim 1 wherein the alloy matrix is a cobalt aluminum alloy.

5. A composite abrasive body consisting of cubic boron nitride crystals and an alloy matrix, said alloy matrix being tool steel.

6. The compact abrasive body of claim 5 wherein the tool steel is molybdenum toolste el. 

1. AN ABRASIVE BODY CONSISTING ESSENTIALLY OF CUBIC BORON NITRIDE CRYSTALS AND AN ALLOY MATRIX, THE ALLOY MATRIX COMPRISING A FIRST METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, SILICON, VANADIUM, NIOBIUM, LANTHANUM AND OTHER RARE EARTH METALS AND TOGETHER THEREWITH ANY METAL THAT, WHEN MOLTEN, WILL WET SAID FIRST METAL AND FORM AN ALLOY THEREWITH THAT EXHIBITS FINITE, LIMITED REACTIVITY WITH CUBIC BORON NITRIDE AND IS HOMOGENEOUS ON SOLIDIFICATION, SAID ALLOY HAVING A MELTING POINT BELOW ABOUT 1500*C AND EACH COMPONENT OF SAID ALLOY CONSISTING OF AT LEAST 1 PERCENT BY WEIGHT OF SAID ALLOY AND SAID ALLOY MATRIX CONSTITUTING AT LEAST 30% OF VOLUME OF SAID ABRASIVE BODY.
 2. The abrasive body of claim 1 wherein the body is formed as a tool insert.
 3. The abrasive body of claim 1 wherein the alloy matrix is a nickel aluminum alloy.
 4. The abrasive body of claim 1 wherein the alloy matrix is a cobalt aluminum alloy.
 5. A composite abrasive body consisting of cubic boron nitride crystals and an alloy matrix, said alloy matrix being tool steel.
 6. The compact abrasive body of claim 5 wherein the tool steel is molybdenum tool steel. 